The Units field carried a Default of "metric" and had no blank option, so
the global panel layer always set a value and locked organizations and
users out of changing it. Mirror the fix already applied to OpenSky's plan.
- Plugin descriptor: drop the "metric" Default and add a blank "Not set"
option; runtime still falls back to metric when no layer sets it.
- Web App: add a "Not set" choice to the Units control and stop forcing
metric back into the form on load.
- Test guards that the units field has no Default and offers a blank option.
- Rebuilt embedded frontend.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
